The present invention relates to a method and apparatus for detecting normal and angled cracks on or beneath a surface using infrared thermal imaging where the crack plane is perpendicular to or has a perpendicular component to an imaged surface.
Normal cracks in which the crack plane is perpendicular to the surface are major safety and reliability concerns for structural components used in airplanes, automobiles, pipes in nuclear reactor systems, and others. Normal surface breaking and subsurface cracks, for example, resulting from fatigue at locations of high stress concentrations and from corrosion in pipes in nuclear power plant piping, are not effectively detected by known nondestructive testing techniques. Considerable research and development works have and are still being devoted for detecting and characterizing these cracks.
Most of the current developments utilize ultrasonic techniques. Depending on the configurations of the cracks, various ultrasonic systems can be used. For example, angled ultrasonic waves can be used for detecting internal cracks, and surface waves for surface breaking cracks. This type of techniques typically requires contact scanning of sample to obtain information of crack distribution which may be difficult for components of complex geometry and the testing is usually time consuming.
Infrared thermal imaging, traditionally known as thermography, is becoming widely used in nondestructive detection of defects in components such as developed by Jiangang Sun, Chris M. Deemer, and William A. Ellingson. Most thermal imaging techniques are based upon applying flash heating on a sample surface, then monitoring the decay of the surface temperature by an infrared camera. Defects on or under the surface can then be found from the thermal images of the surface because defects have different thermal resistance than the sample material and therefore affect the decay rate of the local surface temperature. Depending on the direction of heat transfer, lateral cracks in which the crack plane is parallel to the sample surface are easily determined by applying instantaneous uniform heating on the entire surface, while a single normal crack can be determined by applying the same heating on only one side of the crack, but requires prior knowledge of the crack direction to be aligned with experimental setup and can only characterize one crack at a time.
Recently a new hybrid ultrasonic/thermal-imaging technique called ThermoSonix was developed by Indigo Systems to detect cracks in a component. In this hybrid ultrasonic/thermal-imaging technique, a short pulse of ultrasound energy is applied to a component, which causes frictional heating at crack surfaces that may reach to the surface monitored by an infrared camera. This technique can detect cracks of all orientations. The technique is based on the frictional heating between the two interfaces of a tight crack so frictional heating can be generated from the two surfaces on either side of the cracks. However, the frictional heating does not exist when the crack surfaces are separated by a small gap in open cracks whose two side surfaces are not in contact. Such open cracks are more severe but cannot be detected by this technique.
Known techniques cannot reliably detect and accurately characterize a distribution of normal cracks. Known non-destructive evaluation (NDE) techniques cannot reliably detect and accurately characterize cracks normal to the surface in ceramic materials.
A principal object of the present invention is to provide a method and infrared thermal imaging apparatus for detecting normal and angled cracks on or beneath a surface where the crack plane is perpendicular to or has a perpendicular component to an imaged surface.
It is another object of the invention to provide such method and infrared thermal imaging apparatus for detecting normal and angled cracks on or beneath a surface that is generally fast, and accurate.
It is another object of the invention to provide such method and infrared thermal imaging apparatus for detecting normal and angled cracks on or beneath a surface for automated non-destructive evaluation (NDE) thermal imaging of samples formed of various materials and that is not dependent on material properties.
It is another object of the invention to provide such method and infrared thermal imaging apparatus for detecting normal and angled cracks on or beneath a surface substantially without negative effect and that overcome many of the disadvantages of prior arrangements.
In brief, a method and infrared thermal imaging apparatus are provided for detecting normal and angled cracks on or beneath a sample surface using infrared thermal imaging where the crack plane is perpendicular to or has a perpendicular component to an imaged surface of the sample. A constant heating source is used for heating a section of the sample to produce a lateral heat transfer through the sample. An infrared camera is positioned near one side of the sample for receiving thermal image data resulting from the lateral heat transfer through the sample. A data acquisition and processing computer is used for acquiring and differentiation processing thermal image data from the infrared camera for generating a two-dimensional image to detect the normal and angled cracks.